Integrate video chat into app.js (as an async component)

Summary:
This integrates two apps into one, making things simpler and convenient.
The app.js size does not grow because the whole Room component as well as openvidu lib
are loaded when needed.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: vanmeeuwen, #hkccp_developers

Differential Revision: https://git.kolab.org/D2014